1. Field of the Invention
The invention relates generally to electrical switching apparatus and, more particularly, to latch assemblies for electrical switching apparatus, such as circuit breakers. The invention also relates to latch engagement control mechanisms for circuit breaker latch assemblies.
2. Background Information
Electrical switching apparatus, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, abnormal voltage and other fault conditions. Typically, circuit breakers include a housing, and an operating mechanism which opens separable electrical contacts to interrupt the flow of current through the conductors of an electrical system in response to such fault conditions as detected, for example, by a trip unit. The operating mechanism can also be employed to manually open and close the separable contacts, for example, by manipulating an actuator such as, for example and without limitation, a push button disposed on the exterior of the circuit breaker housing.
The operating mechanisms of some low-voltage circuit breakers, for example, typically include both a closing assembly and an opening assembly that are structured to close (e.g., contacts electrically connected) and open (e.g., contacts separated), respectively, the separable contacts. Specifically, the operating mechanism includes a pole shaft, a number of stored energy devices such as, for example, an opening spring and a closing spring, and a number of links coupled to the pole shaft. Typically, there are at least two links which act cooperatively as a toggle assembly. When the separable contacts are tripped open, the toggle assembly is disposed in a collapsed configuration and, when the separable contacts are closed, the toggle assembly is disposed in a second, straightened position in which the toggle links are generally aligned. The opening spring biases the pole shaft toward the open position, which tends to collapse the toggle assembly. A latch assembly, which is also spring-biased, maintains the toggle assembly in the second position. The toggle assembly, in turn, holds the pole shaft in the closed position. More specifically, the latch assembly is coupled to the toggle assembly either directly or via a number of additional links. When an electrical fault condition occurs, the latch assembly is released, allowing the pole shaft, which is driven by the opening spring to cause the toggle assembly to collapse. Thus, if the toggle assembly is collapsed, the pole shaft can rotate, thereby moving the separable electrical contacts into the open position. Alternatively, the opening operation can be initiated manually, for example, by pushing the aforementioned push button on the circuit breaker housing. Specifically, depressing the push button manipulates the latch assembly which, in turn, releases the toggle assembly causing it to collapse and permit the pole shaft to pivot to open the separable contacts. The latch assembly is operable in a similar manner with respect to the closing assembly.
The latch assembly typically includes a pin member having a flat portion, and a planar pivotal member, sometimes referred to as the “latch plate.” The latch plate is structured to engage the pin member unless and until the pin member is sufficiently pivoted for the flat portion to provide a clearance between the latch plate and the pin member. When such clearance is provided, the latch plate may pivot (e.g., swing), thereby releasing the toggle assembly to collapse. The distance from the contact point where the latch plate makes contact with the pin member to the flat portion of the pin member, is referred to as the “latch engagement.” The latch engagement and, in particular, maintaining a consistent latch engagement, is critical to performance of the circuit breaker. For example, if the latch engagement is too large, the pin member requires too much work to pivot enough to allow clearance for the latch plate to swing and, if the latch engagement is too small, premature, unintentional release of the latch assembly could result. More specifically, if the latch engagement is too small, minor variations or vibrations or other disturbances could undesirably cause such unintentional release. Also, if the latch engagement is small, the forces and associated stress on the relatively small piece of material that is maintaining the engagement, are significant, and could cause the piece to fail (e.g., become deformed; break).
Among the disadvantages with known latch assemblies is that the latch engagement is sometimes inconsistent. This is, in large part, attributable to dimensional tolerance variations among the components of the latch assembly. Specifically, each component has its own set of dimensional tolerances. When the components are assembled, variations in the tolerances can accumulate or “stack” up across the assembly, and thereby adversely affect the latch engagement. Additionally, known latch assemblies typically employ a number of pins, protrusions, or other suitable features to establish and maintain the desired position of the latch pin with respect to the latch plate, and thereby establish the latch engagement. Such features (e.g., pins; protrusions), are typically disposed on a component or portion of the circuit breaker that is separate from the latch pin and/or unrelated to the interaction of the latch pin with the latch plate. Consequently, the latch engagement is compromised because misalignment can occur, for example, from assembly or manufacturing errors, or by the aforementioned stacking of dimensional tolerances across the components of the assembly. As a result, circuit breaker performance suffers.
There is, therefore, room for improvement in electrical switching apparatus, such as circuit breakers, and in latch assemblies and in latch engagement control mechanisms therefor.